This invention relates to unloading means for unloading bulk material from storage spaces such as cargo holds. Bulk material is material with various grain diameters such as corn, coal, fertilizer, portland cement and similar materials.
Known self unloading means for unloading of bulk material in ships employ scraper conveyors attaching the bulk material from above and moving it in one of two main horizontal directions. Due to the fact that the width of attack will be limited to the width of the individual scraper, it will also be necessary to be able to move the scraper conveyor vertically, so that it can be lowered at the same rate as the surface of the bulk material sinks due to the unloading thereof. This type of scraper conveyor is generally known from, for instance, FR Patent No. 1,343,751, but also from NO Patent No. 147,376, corresponding to U.S. Pat. No. 4,474,523, where a scraper is arranged vertically movably along one or more of the walls in the storage space.
As a result of the operation of the scraper conveyor, the bulk material will be moved in one of the horizontal directions of the hold and will be collected near one of the walls thereof, from which position the bulk material can be transported out of the hold by suitable elevator means which extends to the bottom of the hold. Known means require use of two scrapers, one in each of the main horizontal directions, and also suitable elevator means. The scraper conveyor is arranged on guide beams extending across the hold. Rails are mounted against the sidewalls of the hold to guide the scraper conveyor when the scraper conveyor is lowered at the same rate as the sinking surface of the bulk material. This arrangement requires a complicated operation and costly control system.
Another type of unloading arrangement is a system where the bulk material is unloaded from the bottom of the cargo hold. This system is known as the "Steven Adamson" system and utilizes hoppers covering and mounted to the floor of the hold. The hoppers are opened and closed by control valves. The bulk material flows through the hoppers and down to a horizontal belt conveyor transporting the material to an elevator. The drawback with this system is that auxiliary equipment has to be mounted on the hoppers to destroy or avoid bridging or clogging of the bulk material because the system will not work if the bulk material becomes clogged or bridged. Furthermore, such system is useless for unloading fluidizable material. The system works due to gravity and is dependent on use of flowable but not fluidizable bulk material. This limits the use of such unloading system.
Both of the above mentioned system, i.e. the system having two scraper conveyors and the system for unloading from the hold bottom by using hoppers, require if mounted in older ships large and costly refitting operations. The "Steven Adamson" system has therefore always been built into new ships, whereas the system for unloading from the top of the cargo requires a hold of rectangular shape adjusted to fit the scrapers. Such systems also have reduced the accessibility for inspection and maintenance during unloading.